Erm, the Phenom 955 is a great cpu... and for price/performance, you really cannot get any better for value.
The i7 is one of the best chips in the world, but if you were to do sandy bridge it will cost you an absolute bomb. I would get the i52500k... unlocked multipler, overclockable to 5ghz on air cooling alone, very very very very fast (like the i72600k but cheaper), great paired up with the GTX570 like yours but expensive and will require a new motherboard.
Motherboard: (If you go Intel)
P67 = Overclocking
H67 = Onboard graphics
Z68 = Overclocking + Onboard Graphics
Get a z68 chipset board so you can take advantage of the onboard graphics, boosting your gpu power by a good 20% and they overclock excellently. This is one I would get:
http://www.overclockers.co.uk/showproduct.php?prodid=MB-476-AS&groupid=701&catid=5&subcat=1990
Pros to intel:
Faster
Better Overclocking
Last longer
Won't have to upgrade any time soon
Just freakin' excellent!!!!
Cons to intel:
Expensive
Requires new motherboard
If you are happy with the frames you are getting then keep with the AMD Phenom, but if you want more frames, a more reliable and faster computer and something that will last for years, upgrade to Intel xD
Hope this helps
Cinderz